This invention is intended for use in the process of drilling a borehole e.g. an oil well, and has to do with a process for fluid detection which could lead to a blow-out. Detection of this event is based on the difference between the inlet and outlet flow rates of the drilling mud injected into the borehole.
When, during the drilling of a borehole, one has passed through an impermeable layer and then reaches a permeable layer containing a liquid or gaseous fluid under pressure, this fluid tends to push its way into the borehole if the column of drilling mud in the hole is unable to counter- balance the pressure of the formation fluid. The latter fluid then pushes the mud upwards: there is then said to be a fluid influx. This produces an unstable condition: as the fluid replaces the mud in the borehole, the average density of the counter-pressure column inside the hole decreases and the imbalance is aggravated. If no steps are taken to correct this, the condition accelerates and leads to a catastrophic blow-out.
It is therefore of the greatest importance that one be able to detect the onset of this condition as early as possible, i.e. when the influx is still at the controllable stage when one can take immediate emergency action which consists of closing the well at the surface using a blow-out preventer.
This detection can be achieved by measuring the inlet flow rate of the mud injected into the shaft, the outlet flow rate of the mud rising from the well and calculating the difference between these two flow rates. When the difference is no longer zero in that the outlet rate exceeds the inlet rate, this marks the onset of instability in the well.
Measuring the inlet flow rate presents no difficulty: one can use a flowmeter, of the magnetic type for instance, or one can calculate the rate from the speed of the pump which causes the mud to circulate, e.g. of the volumetric type. It is more difficult to calculate the outlet rate, however: this measurement actually has to make allowance for the fact that the mud, when it comes out of the well, is soiled and takes the form of a heterogeneous mixture (water, oil, fines, cuttings) of a thixotropic consistency. It is not possible to measure the flow rate of such a mixture to any degree of accuracy using a conventional flowmeter.
The purpose of this invention is to provide early, automatic and reliable detection, at low cost, of fluid influx in wells, by analyzing the difference between the inlet and outlet flow rates of the drilling mud, despite the imprecision involved in the measurement of the outlet flow rate.